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”Characteristics of SAR interferogram?”
  Principle of InSAR / Intermediate Course2

InSAR and GPS, the space geodetic technique which enable us to detect surface deformation, have many similarities such as using the phase of a radio wave but differ as below.

Comparison between InSAR and GPS

Difference between SAR interferometry and GPS measurements in the observation of surface deformation
(A) Observation facilities on the ground Unnecessary Receiver required
(B) 2-dimensional information Possible Impossible
(C) Continuous observation Impossible Possible
(D) Absolute amount of deformation Not obtained directly *1 Observable
(E) Measurand Two-way between a satellite and the ground surface One-way from a satellite to a receiver
(F) Direction of measurement 1-dimentional *2 3-dimentional
(G) Observation time Once every dozens of days 24 hours available

*1:Possible through analysis
*2:Line of sight direction between a satellite and the ground surface

(A) No facilities are required at observation points
GPS works at the point where a receiver is installed but InSAR does not need any facilities on the ground.
(B) 2-dimentional information is gathered in a moment
InSAR allows us to observe a wide area at one time and get high-density information. If the radar is installed on a satellite, it measures the area which is a square tens of kilometers on a side in about ten seconds. Observation data is gathered almost instantaneously.
(C) Continuous observation is impossible
One time observation by one SAR antenna can provide a SAR image, but a pair of SAR data is required to create an interferogram. For the satellite based InSAR, using the radar installed on a satellite which orbits the earth on the same track, two data at distinct times are acquired . Because the two data have to be acquired by the satellite with the same condition (all sorts of measurement parameters including a frequency), these are generally observed by the same satellite. However, the twin satellite with the same parameters like European satellites ERS-1 and ERS-2 is also used.
(D) Absolute amount of deformation is not obtained.
Two observation data acquired at distinct times provide the difference of the surface position between two observations, but not the absolute position of the surface. It is the relative amount of deformation that is provided in a SAR image. We need to assume that there is no displacement in the area so far away from a fault or to use a different kind of observation data to measure the absolute amount of deformation.
(E) Two-way measurement between a satellite and the ground surface
GPS measures the one-way distance from a satellite to a receiver on the ground, but SAR measures the two-way distance from the SAR antenna (transmission) to a target (reflection) to the SAR antenna (reception). So the movement of the target and the amount of atmosphere delay are doubled. For this reason, although L-band wavelength of ALOS is 23.6cm, the phases in a SAR image go round (360 degrees) for every the half wavelength of 11.8cm due to propagation delay or surface deformation. Also the surface movement is conventionally showed in terms of real one-way distance.
(F) InSAR can measure surface deformation only in the LOS direction
GPS measurements can provide 3-dimentional displacement because of observing from different directions at the same time by several GPS satellites. However, multidirectional SAR observation is hardly done because of a limited number of SAR satellites. In addition, SAR can measure the surface deformation in the LOS direction between the satellite and the surface. InSAR requires two observations which is done by the satellite on almost the same orbital positions whose distance must be less than a few kilometers. So if just one satellite is used in measurement, the direction of measurement is often the LOS direction from the same obliquely upward point on the track.
(G) Limited observation time
GPS observation works 24 hours a day, but SAR observation is possible only when the satellite is located above the target area. If just one SAR satellite observes the entire globe, the interval at which the satellite observes the same point is commonly a few tens of days(46 days for ALOS). This means that if the satellite observes a target point, it is a few tens of days later that the satellite can observe the same point with the same condition.


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